This invention relates to a structural spanning member capable of use in constructional engineering structures such as roofings, floorings and walls for buildings and deckings for bridges in which the member is positioned to extend between end supports and which comprises a web and a pair of chords connected to the web and directed mutually oppositely to each other with respect to the web.
Known decking structures comprise girders or other load bearing members spanning between end supports, which members carry cladding of lighter gauge material, usually metal and often steel. Such cladding, which is not itself a structural member, comprises a series of webs and chords formed in a single sheet.
At Salford University in the United Kingdom a roofing structure was proposed in which a zig-zag configuration was obtained by having a series of alternate truncated V-shaped girder chords and inverted truncated V-shaped girder chords of structural rigidity, with each chord formed as a separate member, and a plurality of separate web members extending between each V-shaped chord and its adjacent inverted V-shaped chord, the plurality of web members extending in laterally contiguous relationship from one and the wall to the other, said web members being formed of light gauge metal sheeting, the longitudinal dimension of which, in elevation, is arranged between the chords. The web members were formed from pressed sheet normally available only in cut lengths of several meters. The web members were stiffened so as to assist in the structural function of the structure, hereinafter referred to as "a spanned-chord structure".
When considering the material available to form these spanned chord structures, certain basic data concerning the strip sheeting produced by strip mills in the United Kingdom is recognised. The strip mills produce sheeting in continuous lengths which conveniently are supplied to the finishing manufacturers in coils. It may be finished as pressed sheet in lengths of several meters. This sheeting will be referred to herein as "light gauge" if it has a thickness in a range of 0.6 to 1.4 mm, and as "heavy gauge" if it has a thickness in a range of 1.6 mm and upwards.
In the spanned chord structure, the material employed was steel and the chords were fabricated from folded plate, whilst the web members extending longitudinally between the chords were of light gauge sheeting. These web members of pressed sheeting were up to 2 meters in length whilst utilizing the maximum width of the sheeting available from the rolling mills.
With this spanned chord structure in which both the chords and the web members are structural members, each pitch comprises a first lower truncated upright V-shaped chord joined by a plurality of web members to an upper inverted truncated V-shaped chord itself joined by a plurality of web members to a second lower upright truncated V-shaped chord. The joining operations are effected by the use of self-drilling, self-tapping screws between each web member and its upper and lower V-shaped chords, and between each web member and the web member(s) contiguous with it. Of necessity, for each roof structure of one or more pitches, these steps in the fabrication are carried out at ground level and then that complete roof structure is raised into position; the raising being accomplished by equipment such as hydraulic jacks.
In this spanned-chord structure, the light gauge pressed sheeting has not been employed so that its longitudinal dimension extends across the roof span.
Where in another prior art the longitudinal dimension of the light gauge sheeting extends across the building span, it extends merely as cladding between truss members or portal frames whilst itself being supported by girders or purlins. Thus, previously cold rolled light gauge sheeting has been used solely as cladding or covering for roof spans. The maximum strip width of 1.2 meters is formed with a plurality of laterally extending ridges and employed to cover a plurality of girders or purlins when the longitudinal dimension of the spanning member is also the longitudinal dimension of the sheeting leaving the mill.
In practice, this has resulted in decking sheets which when seen in lateral section have a series of trapezoidal upper and lower chords, with a depending dimension between upper apex and lower apex of usually less than 100 mm. Currently from cold rolling finishing mills producing sheeting in trapezoidal form, this depending dimension varies according to geographical source with British product being a maximum 63 mm, Italian 75 mm and Swedish 100 mm. This cold rolled sheeting itself is capable of being employed as a structural member for roofing between side walls or portal members where the intermediate spacing is only between three and six meters. For most practical roofing purposes the span or spacing to be bridged is 7 meters minimum and may be up to 20 meters.
Other recent proposals for decking structures by Norrbottens Jarnverk AB of Sweden are shown in Swedish Patent Applications Nos. 75 04906-4 dated 28th Apr. 1975 and 75 06556-5 of 9th June 1975. These disclose the use of individual sheeting members, each sheeting member having a configuration which, when seen in lateral section, comprises a single complete trapezoidal section. Such sheeting members are sold inter alia for use as structural spanning members per se. However, it should be appreciated that even these sheeting members are of limited longitudinal span because of the width of sheeting available (1.2 meters) is contoured to provide the whole member. The whole member comprises a pair of laterally projecting lower chords for connection to adjacent members, and a pair of webs each upwardly directed from a respective lower chord and an upper chord extending between the webs. The upper chord is strengthened by laterally extending embossing or indentations and the webs are stiffened along their length by longitudinally extending ridges. Since the rigidity of the sheeting is primarily determined by the spacing between the upper chord and the lower chords, the fact that the upper chord is approximately 550 mm of the available width of 1.2 meters still places a limitation on the unsupported span of these single trapezoidal-sectioned structural members.
The reason is that whilst the embossing of the upper chord increases bending resistance due to cladding superimposed on the chord, the embossed part of the chord has to be disregarded in calculating the unsupported span for which the member may be used.
The present invention has the object of providing a structural spanning member which from the available widths of sheeting is designed to maximize the unsupported span which the member may bridge.
An additional but related object is to permit the avoidance of the use of purlins and other intermediate support members to achieve economy in building costs.
A further object is to provide a structural spanning member which together with like members provides a weather proof roof which does not of necessity require additional water proofing with bitumens, felt or asphalt.